AN IAI1124 FROM TKOF; CLB TO FL230 EXPERIENCED LOSS OF COM CAUSED BY A FAILED SWITCH ON THE FO'S YOKE.

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AT XA58; APR/FRI/05; WE DEPARTED ZZZ ON AN IFR FLT TO OUR DEST; A BRIEF 50 MINS AWAY. THE PREFLT; TAXI; TKOF CHKS AND COMS WERE ALL NORMAL UNTIL ATTEMPTING TO CONTACT DEP CTL. UPON CALLING DEP; WE BRIEFLY ENCOUNTERED AN AUDIO FEEDBACK MAKING THE ASSIGNED HDG AND ALT DIFFICULT TO UNDERSTAND. TO THE BEST OF MY RECOLLECTION; THE CTLR REQUESTED THAT WE READ BACK THE ASSIGNED HDG AND ALT A SECOND TIME. THE ANNOYING AUDIO SIT SUBSIDED MOMENTARILY; AND WE RECONFIRMED OUR CLRNC INCLUDING THE COMMENT THAT SOMEONE HAD A STUCK MIKE. WE IMMEDIATELY INVESTIGATED THE POSSIBILITY THAT THE STUCK MIKE MAY BE OUR OWN. ACCORDINGLY; WE VERIFIED ALL AUDIO SWITCHES AND SELECTOR POS; AND ULTIMATELY SWITCHED XMITTERS. DURING A PERIOD OF IMPROVED COM; WE COPIED OUR HDOF TO ZHU. AGAIN; WE ENCOUNTERED THE ANNOYING FEEDBACK AND WERE ADVISED BY THE NEW CTLR THAT SOMEONE HAD A STUCK MIKE. ALMOST SIMULTANEOUSLY ANOTHER FLT CREW COMMENTED THAT THE CTLR'S XMISSION WAS BLOCKED. AFTER SEVERAL COMS WITH OTHER FLTS; THE CTLR RETURNED TO US AND ADVISED THAT HE COULD HEAR OUR COCKPIT CONVERSATION; CONFIRMING IT WAS OUR ACFT WITH THE AUDIO PROB. WE CONFIRMED OUR NEW CLRNC TO FL230; AND ADVISED THE CTLR THAT WE HAD EXPERIENCED AN AUDIO SIT ON A PRIOR FREQ. THE PF CONTINUED CLBING AND LEVELED OFF AT OUR ASSIGNED ALT OF FL230. AS I WAS THE PNF; I CONTINUED TO INVESTIGATE ALL OPTIONS OF ISOLATING THE AUDIO FEEDBACK. THESE INCLUDED TEMPORARILY SWITCHING FROM HEADSETS TO HAND MIKES; CONFIRMING BOOM MIKE VERSUS OXYGEN MIKE SELECTOR SWITCHES; UNPLUGGING THE OXYGEN MASK; ISOLATING THE FLT DECK INTERPHONE SYS; CHKING SQUELCH; VERIFYING VOLUME CTLS; YOKE MIKE SWITCHES AND AGAIN SWITCHING BTWN OUR NEWLY INSTALLED COLLINS XMITTERS. FOR A BRIEF MOMENT; I CAUGHT A GLIMPSE OF THE GREEN XMITTER LIGHT ON THE VOX INTERPHONE CONTROLLER WHICH IS POSITIONED ON THE THROTTLE PEDESTAL. THE LOW ALT SECTOR CTLR THEN HANDED US OFF TO A HIGHER ALT CTLR WHO CLRED US TO FL260. UPON LEVELING OFF AT FL260; OR SHORTLY THEREAFTER; THE CTLR ADVISED THAT HE WAS NOT RECEIVING OUR MODE C. I INFORMED HIM THAT WE HAD EXPERIENCED AVIONICS DIFFICULTIES PRIOR TO HDOF AND THAT I WOULD SWITCH TO OUR #2 XPONDER. APPROX 90 SECONDS LATER; I INQUIRED IF OUR MODE C APPEARED TO BE OPERATIVE. THE RESPONSE WAS AFFIRMATIVE. THE REMAINDER OF THE FLT TO OUR DEST AND THE RETURN FLT TO ZZZ WAS ROUTINE AND WITHOUT INCIDENT. IMMEDIATELY UPON COMPLETING OUR SHUTDOWN CHKS; WE VISITED THE LCL AVIONICS FACILITY AND REQUESTED A THOROUGH INVESTIGATION OF OUR AUDIO SYS; BOTH XPONDERS AND THE AUTOPLT. THE AUDIO COMPLICATION WAS DUPLICATED AND DETERMINED TO BE AN INTERNAL MALFUNCTION OF A MIKE SWITCH ON THE COPLT'S YOKE. A NEW SWITCH WAS INSTALLED AND HAS RESOLVED THAT PROB. AVIONICS PERSONNEL WERE UNABLE TO DUPLICATE THE MODE C FAILURE. HOWEVER; THEY SUGGESTED THE ANTENNAE MAY HAVE BEEN TEMPORARILY BLOCKED BY THE FUSELAGE POS. BOTH XPONDER ANTENNAE ARE LOCATED ON THE LOWER FUSELAGE CTRLINE. THE AUTOPLT ALSO PERFORMED SATISFACTORILY DURING A GND TEST.